The present invention relates generally to optical communications systems and methods, and more particularly, to the use of bandwidth efficient modulation techniques to transmit information over an optical link.
Previous solutions for transmitting the maximum amount of information on one optical carrier in an optical link involved increasing the speed of simple on-off-keying (OOK) up to the limitations of available electronics technology. For on-off-keying, the bit rate is directly proportional to the electrical bandwidth of the electronics (1 bit/Hz). For example, 40 Gbit on-off-keying requires 40 GHz electronics at least. Not only are such electronics difficult to manufacture, they are expensive, thermally sensitive, and subject to exacting design techniques in every way. At about 40 GHz, some fundamental physical constraints become a problem; the unity gain frequencies of currently available IC foundry transistors are not adequate. Extremely advanced techniques that are currently barely achievable in the laboratory are required for greater than 30 Gbit on-off-keying. These devices are thermally unstable.
Various patents have been issued relating to transmission of data via optical communication systems that were uncovered in a search relating to the present invention. These patents disclose apparatus and methods relating to optical modulators, intermodulation products, amplifiers, receivers, heterodyne communication frequency multiplexed carriers, linearizers, signal sources, frequency division multiplexing, intensity modulation, mixers, spread spectrum CDMA systems, harmonic interference, optical carrier filtering and polarization schemes. However, none of the patents uncovered in the search disclose the use of bandwidth efficient modulation techniques to transmit information over an optical link.
Therefore, it would be advantageous to have systems and methods that improve the maximum data transmission capacity on one optical carrier in an optical link.
The present invention increases the maximum data rate capacity per optical carrier on an optical link. The present invention intensity modulates an optical carrier with one or more (microwave) carriers that have been higher order modulated (M-ary ASK, PSK, QAM, etc.). These higher order modulation formats are known as bandwidth efficient modulation (BEM) methods, because they encode more than one bit per encoded symbol, but the electrical bandwidth used is proportional to the symbol rate. As a result, as the number of bits/symbol is increased, the bit rate increases without an increase in occupied bandwidth. Thus, 2 bit/Hz or more of optical channel bandwidth may be transmitted. In contrast, on-off-keying is limited to 1 bit/Hz.
An exemplary system comprises a plurality of modulators that modulate a plurality of data streams. The first modulator may be an amplitude shift keyed (ASK) modulator, using either on-off-keying or multiple amplitude steps. The remaining modulators encode their data streams on (microwave) carriers using bandwidth efficient modulation.
A plurality of frequency converters are then used to convert the carriers to a plurality of frequencies that are respectively spaced such that the filters at the receiver may cleanly separate their associated data spectral lobes. A combiner is used to combine the signals into a composite (microwave) spectrum that is transmitted over the optical link. At the receiving end of the link, a splitter and a plurality of bandpass filters are used to separate the recovered spectrum into its individual data streams. A plurality of frequency converters and a plurality of demodulators are used to extract the originally transmitted data from each data stream.
As described above, each of the bandwidth-efficient-modulated carriers can transmit multiple bits per Hz of optical link bandwidth. Although there is some wasted band in separating the individual BEM spectra of the composite spectrum, the aggregate rate for the link is higher than 1 bit/Hz. In addition, the individual bandwidth efficient modulated signals use digital switching electronics proportional to their relatively slow symbol rate, so the maximum aggregate link data rate is not limited by the transistor technology speed. Rather, the aggregate link rate is limited by the optical channel electrical bandwidth, which today may be 40 GHz. Thus, because the present invention achieves over 1 bit/Hz spectral efficiency, over 40 Gbit may be transmitted on one optical carrier.
An exemplary method comprises the following steps. A plurality of data streams are encoded for transmission onto multiple copies of a microwave carrier to produce multiple signals having a first carrier frequency. The carriers are converted to appropriate frequencies that are spaced so that their associated data spectral lobes may be separated. The frequency converted modulated carriers are then combined to provide a composite signal. The composite signal is transmitted over the optical link. The composite signal is reproduced at a receiving end of the optical link. The individual data streams are separated out from the composite signal, and are converted back to the original carrier frequency. The data streams are then demodulated to generate the originally transmitted data.